The saying, "a man's reach should exceed his grasp," may be inspirational, but it certainly is not ergonomic when considering assembly line work design.
Assembly work involves considerable reaching and grasping. But do not forget that lifting, turning and twisting are part of the work as well. These are motions that take place in typical production operation involving human workers and all of it is potentially harmful if the work is not designed properly.
In the race against the clock, manufacturing operations have found that the motions required by manual assembly take a considerable toll on the body. The time pressures and repetitiveness competitive production demands intensify the effects of these movements, adding up to create a host of musculoskeletal disorders (MSDs), with workers suffering from these particularly in the low back and upper extremities.
For both companies and their workforces, the impacts of MSDs are not small, affecting about 1 million workers and costing business between $45 and $50 billion annually in compensation, lost wages and decreased productivity according to a National Academy of Sciences (NAS) report. For the employee these injuries can make their working life painful and many times bring it to an early end.
Despite the absence of an ergonomics regulations, smart businesses recognize the problems and the benefits of a continuous and in-depth ergonomics program, working towards workplace improvements to relieve bodily stress. Engineering solutions and eliminating body stresses are ways to reduce the frequency of MSDs.
Workstation Analysis
Much of the body stress in assembly is introduced when workers have to move parts or product onto the assembly line, or take them off for packaging, shipment or to move onto the next part of the production process. Even moving a part weighing just a pound every few minutes can add up to tons of exerted force through the course of a shift, having detrimental effects on the body if force is applied in an unnatural fashion.
Keeping the assembly line healthy means keeping the on the job motions within the ergonomic window -- the range of body movement in which a worker can safely operate.
As a basis for workstation analysis, the ergonomic window provides measured limits and encompasses the following distance for the hands from the floor when standing:
- Heavy work -- 28-37 inches
- Light assembly -- 34-38 inches
- Precision work -- 38-46 inches
The ergonomic window also specifies a suggested maximum reach distances for frequently used items when standing:
- 14 inches for two handed tasks
- 18 inches for one-handed task
Lift Tables
Along with conscious efforts to design jobs to relieve these strains, manufacturers are turning to ergonomic equipment solutions to make workstations easier on the bodies and minds of the workers. Powered lift tables provide the versatility manufacturers need to fight against MSDs while keeping pace with the demands for productivity.
The function of the lift table is to keep assembly line workers as close to the work as possible by allowing adjustments as the line operates through the shift.
When it comes to combating MSDs in the workplace, the worker doing the actual job is ultimately the best expert on how the job can be done. The adjustable lift table work surface height provides that vital element of control.
Use of a table lift on the assembly line ties in closely with the principle of the lever from basic physics. Recall that the further the distance the load is from the pivot point (the back, shoulder or elbow in this case) the more work forces needed to move the load.
The key benefit of lift tables is they enable a variety of working positions to minimize movement. Furthermore, powered lift tables prevent the worker from doing any lifting from exceeding the body's lifting capacity. Table adjustability decreases the distance or height over which objects must be moved, and on most standard models, the table power unit can be controlled either using a hand remote or foot control.
Many operations rotate jobs or have fill-in workers during breaks to enable uninterrupted assembly flow. Lift tables provide a work surface that is adjustable to match the size of the operator.
Nevertheless, the main reason for using lift tables is to present the work so the parts or product are within the ergonomic window. Towards that goal, lift tables offer the following design options:
- Load capacity -- weight of the load and if off-center or shifting loads need to be accommodated.
- Load Being Tilted -- lift platforms can be designed to handle boxes, pallets, wire baskets, corrugated steel and other containers.
- Size and Distribution of Load -- platform sizes can be specified to match the size of the container. For loads that are beyond the reach of the ergonomic window, lift table platforms can rotate or tilt. For lift applications, the lift table should be lagged to the floor.
As an additional safety feature, the lifting mechanism beneath the table can be covered with a protective bellows.
Handling Units
Multi-stage container handling units are also being used by assembly lines for easy access to production parts.
The over/under device enables these often-heavy containers to be staged for production or loading without any manual lifting or positioning.
The device allows a smooth flow of the assembly line, while minimizing the reaching distance. Containers are brought into place by forklift at the ready stage. When the operator is ready to access the contents of the container, a pull on a level brings the container into the proper working height and tilts the box for easy access to the parts.
In the meantime, the forklift can bring another container to the ready position. When the operator is finished with the container, a pull on the lever moves that case to the back of the device, ready for pick-up by forklift. In the meantime, the container in the ready position can be moved to the working height.
The over/under workstation eliminates bending motion to avoid lumbar stress. The unit also reduces the horizontal lifting distance to remove or place the part by bringing the container closer to the work.
This action reduces predicated spinal disc compression force. By enabling the load to be moved horizontally to the spine, the amount of weight the worker can safely handle increases. The angle provided by the tilting of the container reduces the mechanical contact pressure stress on the arms while reaching into the container.
In addition, anyone working in proximity to a moving forklift is at risk. As the forklift accesses the device at one end while the operator works it from the other, the unit separates machine and man by as much as six feet.
Today's managers are learning that just speeding up the assembly line will not push productivity. More and more, they are recognizing that making motions fit within the framework of the ergonomic window completes the production picture. Equipment selection is an important tool that significantly augments an on-going program to examine and improve the work process for minimizing MSDs.
Dana Root has had more than 10 years as an ergonomics consultant to business and industry. She holds a master's degree in industrial hygiene with a major in ergonomics from the University of Wisconsin-Milwaukee and a certificate in physical therapy from the University of Texas Southwestern Medical School in Dallas. She can be reached by e-mail at [email protected].
This article made possible with the help of Topper Industrial and Prestolifts.
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